Low-ish progesterone 6DPO

Dana • We have one amazing 5yo IVF baby. 41 and TTC #2. After 4 retrievals with 6 failed transfers, we're doing a few Hail Mary IUIs in Feb/March to use up our donor sperm, then closing the book on TTC 🤞
I had my post-<a href="https://glowing.com/glow-fertility-program">IUI</a> progesterone test today (a day early at 6DPIUI because we're going on holiday) and it was only 16. The nurse says they like it to be over 20, and I have to go and repeat it tomorrow. I'm worried now!
Anyone know about these things? Could it just be because I got the test a day early?